1. The stage-specific responses of eruptive populations to ecosystem alterations have received little attention, despite the relevance of the issue for conservation biology and pest management. This study examined the hypothesis that forest thinning treatments affect the outbreak cycles of Neodiprion abietis , a sawfly defoliator.

2. The densities of eggs, early instar larvae, late-instar larvae, cocoons and adults of N. abietis were monitored during an outbreak in a thinned and an adjacent untreated stand at each of three sites in western Newfoundland, Canada.

3. The amplitude of population fluctuations was greater in thinned than in adjacent untreated stands at certain points in the increasing or peaking stages of N. abietis outbreaks for all life stages sampled. However, the timing and magnitude of differences in fluctuations attributed to thinning varied among the life stages of N. abietis.

4. The largest difference between stand types occurred in the generation preceding peak egg density, when adult densities were three times higher in thinned than untreated stands. Conversely, only moderate differences in egg density between stand types were observed in the following generation. Combined with the observation that the modes of population fluctuations occurred earlier in thinned stands, this suggests that adult dispersal resulted in a redistribution of eggs between thinned and untreated stands.

5. Synthesis and applications. To our knowledge, this is the first study to show that silvicultural practices involving a reduction in forest stand density may alter the outbreak cycles of an eruptive population, potentially increasing the severity of outbreaks in treated, as well as in adjacent untreated, stands. Hence, this study emphasizes the need to consider proactively the potential impact of silvicultural practices on species such as N. abietis, which are most abundant in low-density stands. This field study also demonstrates that the response to ecosystem alteration of an outbreak population can vary with the different life stages of the studied organism, indicating that the detection of the
impact of ecosystem alterations on a population can depend on the life stage sampled.